Wireless communication systems are widely deployed to provide various communication content such as voice, video, packet data, messaging, broadcast, etc. These wireless systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA) systems.
A wireless communication system may include a number of base stations that can support communication for a number of user equipments (UEs). A base station may include multiple transmit and/or receive antennas. Likewise, UEs may include multiple transmit and/or receive antennas. UEs may transmit in the uplink (UL) using transport block assignments indicated by the base station. In conventional wireless systems, such as the currently agreed upon version of the Long Term Evolution (LTE), the transmission in the uplink direction may be performed either as a single input, multiple output (SIMO) or as a closed loop MIMO transmission. With the introduction of UEs with up to four antennas in the LTE-Advanced (LTE-A) specification, additional uplink transmission modes may improve performance.
In MIMO wireless communication systems, it is often desirable for a UE to support spatial multiplexing wherein the UE transmits on at least two transmit signal layers. Doing so requires that it has at least two independent transmitters. Additionally, in some operating modes, it is desirable to transmit the same signal with different phases, i.e., without spatial multiplexing, on multiple independent transmitters. For example, data transmitted by a first UE transmitter and corresponding first transmit chain is transmitted with a first phase and data transmitted by a second UE transmitter and corresponding second transmit chain is transmitted with a second phase. To optimize uplink transmissions, the second phase may be offset from the first phase by one or more 90 degree increments (e.g., 90 degrees or 180 degrees). The difference in phase between a first UE transmitter and a second UE transmitter may be referred to as “phase difference” or “relative phase.” To be most effective, the phase difference or relative phase should be held constant or near constant, referred to as maintaining relative transmit phase continuity, during uplink transmissions. A change in phase difference or relative phase may be referred to as “phase drift” or the inability to maintain relative transmit phase continuity. It is desirable to maintain relative transmit phase continuity even if the relative phase value is not one of the aforementioned 90 degree increments or not any predetermined value. As long as the relative transmit phase is substantially constant, the base station is able to direct the UE to adjust the relative transmit phase to become closer to one of the desired values. When the UE is not able to maintain relative transmit phase continuity, however, the transmit phase adjustment looses effectiveness.
It is often burdensome to require that a UE maintain relative transmit phase at a constant or near constant value. Several factors make it difficult for a UE to do so. Improper transmitter calibration may allow one or more of the transmitters to change in phase over time. Similarly, differences in the respective transmit chains may introduce changes in the relative transmit phase. Further, UEs are often manufactured with an emphasis on reducing size and cost. As a result, UEs often lack circuitry and/or software necessary to properly initialize or keep track of the phase of each transmitter.
Phase drift may also occur between UE transmitters when the transmitters transmit data at disparate powers. According to a common implementation, transmitter power is changed in stages by an analog gain switch. During operation, a UE transmitter may transmit data on a first channel (e.g., the Physical Uplink Shared Channel (PUSCH)) followed in time by a transmission on a second channel (e.g., the Sounding Reference Signals (SRS)). Because the power requirements for each channel are often different, the power of the first and second transmissions in time may be amplified according to a different number of gain stages. This introduces a relative phase change, particularly where the transmitters switch between channels (i.e., are subject to a changing number of gain stages) over time. If the phase changes in the individual transmit chains were identical, then the relative phase would be constant irrespective of the changes over time and over channels. However, the individual transmit chains are usually not identical due to design differences or part to part variations, therefore the relative transmit phase continuity will not be maintained.
One approach to enable a UE to maintain relative transmit phase continuity involves manufacturing a UE according to more stringent design specifications. Such an approach may require constructing large calibration tables, determining phase as a function of temperature, age, power, and the like. This approach is undesirable because it is both computationally burdensome and expensive.
In view of the above, it is desirable to provide wireless communications that avoid the expense and burden of requiring that a UE maintain phase difference between its transmitters, but instead, adopt a communication format utilized with respect to the UE according to a UE's ability to maintain relative transmit phase continuity (whether limited or not). This is desirable for a number of reasons including the fact that base stations in communication with UEs (e.g., evolved Node Bs) are generally equipped with relatively sophisticated hardware and software, and therefore, are better suited to make adjustments to optimize communications according to a UE's perceived abilities.